🧲 For the first time and by mistake, the magnetic fields of seven exoplanets measured

By measuring the winds blowing on seven exoplanets, astronomers accidentally detected their magnetic fields. An unprecedented feat that opens a new way to assess the habitability of these distant worlds.

Earth owes its protection against cosmic radiation to its magnetic field, while Mars, lacking such a shield, lost its atmosphere and water. Until now, measuring these fields around exoplanets seemed impossible.


The researchers used the ESPRESSO instrument on the Very Large Telescope in Chile and MAROON-X on the Gemini North telescope in Hawaii. These spectrographs analyze starlight to reveal the composition of exoplanetary atmospheres. By tracing the motion of iron in these atmospheres, they were able to calculate wind speeds.

Contrary to expectations, the hotter the planet, the slower its winds. This result goes against intuition, as an additional energy input should speed up the winds. The authors of the study published in Nature Astronomy therefore looked for another explanation.

The answer would come from magnetic fields. They act as a brake on charged particles, thus slowing down the winds. The measurements indicate that these fields are on the order of a few gauss, comparable to that of Jupiter in our Solar System.

This discovery allows for the first time to compare the magnetic environment of different worlds. It could guide the search for potentially habitable planets.

Magnetic field and habitability

A planetary magnetic field acts as an invisible shield. It deflects charged particles emitted by stars, called stellar winds, which can erode an atmosphere. Without this protection, a planet can lose its water and become barren, like Mars. Detecting this field around exoplanets is thus important for assessing their ability to host life.

Scientists now have a tool for this: measuring the speed of atmospheric winds. A wind slower than expected indicates the presence of a magnetic field that slows down ionized particles. This technique, discovered and validated on seven hot Jupiters, could be applied to smaller rocky planets in the future.

These findings show that exoplanetary magnetic fields can be equivalent to those in our Solar System. A promising avenue for future searches for Earth-like exoplanets.